There is demand for three-dimensional (3D) glass covers for portable electronic devices, such as laptops, tablets and smart phones. A particularly desirable 3D glass cover has a combination of a two-dimensional (2D) surface, for interaction with a display, and a 3D surface, for wrapping around the edge of the display. The 3D surface may be an undevelopable surface, i.e., a surface that cannot be unfolded or unrolled onto a plane without distortion, and may include any combination of bends, corners, and curves. The bends may be tight and steep. The curves may be irregular. Such 3D glass covers are complex and difficult to make with precision.
For thicker glass sheets, e.g., greater than 0.3 mm, thermal reforming has been used to form 3D glass articles from 2D glass sheets. Thermal reforming involves heating a 2D glass sheet to a forming temperature and then reforming the 2D glass sheet into a 3D shape. Where the reforming is done by sagging or pressing the 2D glass sheet against a mold, it is desirable to keep the temperature of the glass below the softening point of the glass to maintain a good glass surface quality and to avoid a reaction between the glass and the mold. Below the softening point, the glass has a high viscosity and requires a high pressure to be reformed into complex shapes such as bends, corners, and curves. In traditional glass thermal reforming, a plunger is used to apply the needed high pressure. The plunger contacts the glass and presses the glass against the mold.
To achieve a 3D glass article with a uniform thickness, the gap between the plunger surface and the mold surface must be uniform while the plunger presses the glass against the mold. FIG. 1A shows an example of a uniform gap between a plunger surface 2 and a mold surface 4. However, it is often the case that the gap between the plunger surface and the mold surface is not uniform due to small errors in mold machining and alignment errors between the mold and plunger. FIG. 1B shows a non-uniform gap (e.g., at 6) between the plunger surface 2 and mold surface 4 due to misalignment of the plunger with the mold. FIG. 1C shows a non-uniform gap (e.g., at 8) between the plunger surface 2 and mold surface 102 due to machining errors in the mold surface 4.
Non-uniform gaps result in over-pressing in some areas of the glass and under-pressing in other areas of the glass. Over-pressing can create glass thinning that can show up as a noticeable optical distortion in the 3D glass article. Under-pressing can create wrinkles in the 3D glass article, particularly at complex areas of the glass article including bends, corners, and curves. Small machining errors, e.g., on the order of 10 microns, can result in non-uniform gaps that would produce over-pressing and/or under-pressing. Unavoidable thermal expansion of the plunger surface, mold surface, glass, or other equipment involved in the forming can also affect uniformity of the gap. During pressing, the plunger can also stretch the glass so that the thickness of the glass between the plunger surface and mold surface changes. Therefore, even if the gap between the plunger surface and the mold surface is perfect, the stretching of the glass can result in a 3D glass article having a nonuniform thickness. The mold surface or the plunger surface may be designed to compensate for the expected change in glass thickness as a result of stretching. However, this can result in a nonuniform gap between the plunger surface and mold surface, which as noted above can result in over-pressing in some areas of the glass and under-pressing in other areas of the glass.
Accordingly, there is a need for methods of reliably forming 3D glass articles from 2D sheets, particularly where the 2D sheets are formed of ultra-thin, flexible glass having thicknesses of no more than about 0.3 mm.